The present invention relates generally to transistor switching networks, and in particular to a logic-controlled bipolar analog switch.
It is well known in the art to utilize transistor switches in analog signal paths for selectively connecting an analog signal therethrough. One example of the use of such analog switches is the so-called vertical switching circuit of a multiple-channel oscilloscope wherein one of a plurality of signal-processing channels is selectively connected to the vertical deflection system for display. For precise transmission of such signals, it has been popular to use field-effect transistors becasuse when turned on, a signal may be passed from source to drain in an undistorted, although perhaps slightly attenuated, form. However, field-effect transistors have comparatively slow switching speeds, and moreover, field-effect transistors do not lend themselves very well to realization in integrated-circuit form.
One form of analog switch capable of high-speed switching operation and realization in integrated-circuit form is the analog transmission gate taught in U.S. Pat. No. 3,783,307 to David R. Breuer. This so-called analog transmission gate comprises emitter-coupled pairs of bipolar transistors, which, when both transistors of a pair are turned on by supplying emitter current thereto, operate in conjunction with an emitter follower to pass an analog signal to an output terminal. While the bipolar transistor technology permits high-speed switching for the analog transmission gate, fidelity of the analog signals transmitted therethrough is questionable because of gain uncertainties, offset voltages, thermal conditions, and other physical characteristics of the transistor junctions which may result in signal distortion.
Moreover, the digital switch control circuits of the prior art for selecting the appropriate signal path switch are rather complex, requiring many transistors and combinations of logic signals for operation thereof.